The present invention relates to a layered structure having improved laser engravability, to particular embodiments of such layered structures in the form of coextruded films, and to security documents, preferably identification documents, comprising such layered structures.
Inscribing plastics films by means of laser engraving is an important step in the production of film composites. Such film composites play a large part, for example, for security documents, in particular identification documents such as passports, passes, ID cards or credit cards. The black-and-white personalisation of cards by means of laser engraving, that is to say the application of lettering or images such as black-and-white photographs, is generally known. Personalisation by means of laser engraving is generally distinguished in particular by its high security against forgery. The (text) image is formed on the inside of the card, so that it is not possible to remove the (text) image and produce a new (text) image. Separation of the cards into their individual layers in order to reach the laser layer is not possible, for example, in the case of cards made completely of polycarbonate.
In the personalisation of security documents, in particular identification documents, there is an increasing need for improved sharpness and resolution.
EP 190 997 A2 describes various inorganic or organic pigments for laser engraving, which are used in amounts of from 0.001 to 10 wt. % (from 10 to 100,000 wt.ppm), preferably from 0.01 to 3 wt. % (from 100 to 30,000 wt.ppm). In the embodiments described by way of example, the pigments are used in an amount of 1.8 wt. % (18,000 wt.ppm). When such high concentrations of laser-sensitive pigments are used for laser engraving, there is the problem that agglomerates in the laser-inscribable layer lead to so-called “burners”, that is to say thick black points, during the laser inscription and thus impair considerably the quality of the lettering or image to be produced. In addition, in particular when black pigments are used, such high concentrations lead to marked greying of the base material, as a result of which the contrast with respect to the lettering or image to be produced, and hence also its sharpness and resolution, is diminished.
EP 232 502 A2 describes the use of carbon black as a black pigment for the laser engraving of PVC-based identification cards. The carbon black is used in amounts of from 0.1 to 20 g per 100 kg of PVC powder (from 1 to 200 wt.ppm), preferably in an amount of 0.6 g per 100 kg of PVC powder (6 wt.ppm). The above-mentioned problem exists here too with high carbon black concentrations. However, with low carbon black concentrations, which are preferred, the sharpness and resolution of the lettering or image to be produced are not optimal and could therefore be improved.
In EP 1 056 041 A2, it is described that the sharpness and resolution of the laser inscription in a multilayer identification card can be improved if the layer containing the laser-sensitive additive is made as thin as possible, that is to say thinner than 50 μm. EP 1 056 041 A2 gives no indication of a possible influence of the concentration of the additive used. In the single example, an amount of 200 ppm of carbon black, based on a lacquer composition, is used, which after drying results in a markedly higher concentration in the dried layer. The embodiment described in EP 1 056 041 A2 again has the disadvantage of agglomerate formation and increased greying of the base material, as a result of which the sharpness and resolution of the lettering or image to be produced are not optimal and could therefore be improved.
JP 2007-210166 describes a three-layer, laser-inscribable coextruded film of polycarbonate, wherein all three layers must necessarily contain a laser-sensitive additive. However, the presence of laser-sensitive additives in all three layers causes the contrast to be impaired and accordingly, as a result, the sharpness and resolution of the lettering or image to be produced are likewise not optimal and could be improved.
Accordingly, there was a continued need for an improvement in the sharpness and resolution of lettering or images which are to be incorporated by means of laser engraving in order to personalise security documents, in particular identification documents, without having to accept the disadvantage of a poor colour effect because of increased greying of the base material.